1. Field of the Invention
The present invention relates to a chemical mechanical polishing apparatus and a method of chemical mechanical polishing using the same, and in particular to those involving a dresser for refreshing the top surface of a polishing pad for polishing an object to be polished, in which the refreshment is effected by pressure-contacting such dresser under rotation to such polishing pad.
2. Description of the Related Art
Chemical Mechanical Polishing (CMP) apparatus is becoming widely used in the planarization of interlayer insulating film for isolating upper and lower wirings as multi-layered wiring structures are increasingly adopted by system LSIs (Large Scale Integrated circuits). Using the chemical mechanical polishing apparatus allows surface roughness of the interlayer insulating film to be suppressed as small as 100 nm or around.
A known chemical mechanical polishing apparatus is such that being typically shown in FIG. 7. In a chemical mechanical polishing apparatus 10, a polishing pad 14 is stretched on a turn table 12; where the polishing pad 14 being made of a material such as polyurethane foam.
Above the polishing pad 14, a pressurizing head 16 is provided so as to be rotatable and so as to be pressurized against the polishing pad 14. On the bottom surface of the pressurizing head 16, a semiconductor wafer 18 as an object to be polished is held by vacuum chucking so as to orient a plane to be polished downward.
Again above the polishing pad 14 and at a position not overlapped with that for the pressurizing head 16, a dresser 20 is provided so as to be rotatable and so as to be pressurized against the polishing pad 14. A substrate composing the upper portion of the dresser 20 is made of stainless steel (SUS), the under surface of the substrate is nickel-plated, and diamond grains (#100) are embedded in such plated surface.
A nozzle 22 is positioned above the center portion of the polishing pad 14, from which polishing fluid is dropwisely supplied to the center portion of the polishing pad 14. The polishing fluid is spread by the centrifugal force and flows into the interface between the polishing pad 14 and the semiconductor wafer 18, thereby to be used for polishing the semiconductor wafer 18.
The polishing fluid comprises a mixture (slurry) of SiO2 abrasive and a 0.5 wt % KOH solution, and has a primary grain size (size of a single SiO2 grain) of about 40 nm in diameter and an average grain size (size of an agglomerate composed of a couple of SiO2 grains for forming siloxane bonds xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94) of approximately 100 nm in diameter.
Polishing of the semiconductor wafer 18 using such a chemical mechanical polishing apparatus 10 begins with making pressure-contact of the rotating pressurizing head 16 with the polishing pad 14 stretched on the rotating turn table 12, thereby to effect mutual sliding motion between the semiconductor wafer 18 and polishing pad 14 kept under contact. During the polishing of the semiconductor wafer 18, the polishing fluid is constantly dropped from the nozzle 22 and thus supplied to the interface between the polishing pad 14 and the semiconductor wafer 18.
The rotating dresser 20 is also pressure-contacted to the polishing pad 14 stretched on the rotating turn table 12, thereby to effect mutual sliding motion between the dresser 20 and polishing pad 14 kept under contact. This allows constant grinding of the surface of the polishing pad 14 by the dresser 20 so as to keep on creating a fresh surface thereof, which is also referred to as refreshing.
Typical polishing conditions relate to a number of rotation of the pressurizing head 16 of 40 rpm, a pressing force of the pressurizing head 16 against the polishing pad 14 of 58.8 kN/m2, a number of rotation of the turn table 12 of 42 rpm, a number of rotation of the dresser 20 of 34 rpm, a pressing force of the dresser 20 against the polishing pad 14 of 50 N/m2, an amount of polishing of interlayer insulating film of 1,000 nm, a polishing time of approx. 2 min., and a material of the interlayer insulating film of plasma TEOS (P-TEOS).
Next, parameters representing the polishing properties will be described. Such parameters representing the polishing characteristics relate to polishing uniformity (%) and polishing rate (nm/min).
According to a general method for calculating the polishing uniformity, differences in the film thickness before and after the polishing are measured for 49 points on the semiconductor wafer 18, maximum value (Dmax)and minimum value (Dmin) of such differences are found and then further difference between these values is obtained (Dmaxxe2x88x92Dmin), then the value is divided by an average value (Dave) of the differences of the film thickness before and after the polishing measured for the same 49 points multiplied by 2, and the quotient is multiplied by 100, which is expressed by the equation below:
polishing uniformity (%)=(Dmaxxe2x88x92Dmin)xc3x97100/(2xc3x97Dave)
According to a general method for calculating the polishing rate, the average value (Dave: nm) of the differences in the film thickness before and after the polishing measured for 49 points is divided by polishing time (t: min), which is expressed by the equation below:
polishing rate (nm/min)=Dave/t
Now, FIG. 5 is a graph showing a relation between operating time of the dresser 20 and cumulative operating time of the polishing pad 14 for use in the polishing of the semiconductor wafer 18. The figure shows the operating time of the dresser 20 on the abscissa and the cumulative operating time of the polishing pad 14 on the ordinate. It is apparent from the figure that the cumulative operating time of the polishing pad 14 in the polishing of the semiconductor wafer 18 sharply increases as the operating time of the dresser 20 increases.
This is probably because wear of the diamond grains embedded in the surface of the dresser 20 resulted in decrease in the amount of wear of the polishing pad 14, which prolongs the cumulative operating time of the polishing pad 14 used in the polishing of the semiconductor wafer 18. It is generally defined that the lifetime of the polishing pad 14 ends when the thickness thereof reaches a value 0.8 mm thinner than the initial thickness. Lifetime of the dresser 20 ends when the operating time reaches 300 hours.
However in the conventional chemical mechanical polishing apparatus, a problem resides in that, as shown in FIG. 8, the lifetime of the dresser 20 ends too early when the operating time reaches only as short as 100 hours or around, far from 300 hours, since clogs 26 of the polishing fluid are likely to be trapped between the diamond grains 24 on the dresser 20 and thus the polishing performance is ruined. This may result in degraded productivity with the dresser 20 since the dresser 20 needs to be frequently replaced.
Considering the foregoing problems, it is therefore an object of the present invention to provide a chemical mechanical polishing apparatus and a method using thereof, in which the dresser is prevented from being shortened in the lifetime through preventing the clogs of the polishing fluid from being trapped between the diamond grains, and through successfully removing clogs already formed.
To solve the foregoing problem, a chemical mechanical polishing apparatus of the present invention is such that used for polishing an object to be polished while feeding a polishing fluid between the object to be polished and a polishing pad, and comprises a turn table rotating while holding on a top surface of which the polishing pad; a pressurizing head rotating while holding on a bottom surface of which the object to be polished so as to pressure-contact the object to be polished to the polishing pad; a dresser for refreshing the top surface of the polishing pad by pressure-contacting the bottom surface of which to the polishing pad; and a dresser refreshing means for refreshing the dresser during idle period of the dresser.
In such chemical mechanical polishing apparatus of the present invention, the dresser refreshing means preferably refreshes the dresser by immersing the dresser in a refreshing liquid and applying ultrasonic wave to the refreshing liquid.
Again to solve the foregoing problem, a method of chemical mechanical polishing of the present invention is such that having a step for polishing an object to be polished held facedown under rotation by pressure-contacting the object to be polished to a polishing pad held on a turn table while feeding a polishing fluid between the object to be polished and the polishing pad, wherein a dresser in an operating period refreshes the polishing pad by pressure-contacting the bottom surface of the dresser under rotation to the top surface of the polishing pad, and the dresser in an idle period is refreshed while being brought apart from the polishing pad.
In such method of the present invention, the refreshment of the dresser is preferably effected by immersing the dresser in a refreshing liquid and applying ultrasonic wave to the refreshing liquid.
According to the apparatus and method of the present invention, the dresser is refreshed during its idle period while being brought apart from the polishing pad and immersed in the refreshing liquid applied with ultrasonic vibration, so that the dresser is prevented from being shortened in the lifetime through preventing the clogs of the polishing fluid from being trapped between the diamond grains, and through successfully removing clogs already formed.